U.S. patent application number 13/507383 was filed with the patent office on 2013-08-29 for transmission line rf applicator for plasma chamber.
The applicant listed for this patent is Suhail Anwar, Seon-Mee Cho, Jozef Kudela, Ranjit Indrajit Shinde, Carl A. Sorensen, Tsutomu Tanaka, Douglas D. Truong, John M. White. Invention is credited to Suhail Anwar, Seon-Mee Cho, Jozef Kudela, Ranjit Indrajit Shinde, Carl A. Sorensen, Tsutomu Tanaka, Douglas D. Truong, John M. White.
Application Number | 20130221833 13/507383 |
Document ID | / |
Family ID | 49002081 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130221833 |
Kind Code |
A1 |
Kudela; Jozef ; et
al. |
August 29, 2013 |
Transmission line RF applicator for plasma chamber
Abstract
A transmission line RF applicator apparatus and method for
coupling RF power to a plasma in a plasma chamber. The apparatus
comprises an inner conductor and one or two outer conductors. The
main portion of each of the one or two outer conductors includes a
plurality of apertures that extend between an inner surface and an
outer surface of the outer conductor.
Inventors: |
Kudela; Jozef; (San Jose,
CA) ; Tanaka; Tsutomu; (Santa Clara, CA) ;
Sorensen; Carl A.; (Morgan Hill, CA) ; Anwar;
Suhail; (San Jose, CA) ; White; John M.;
(Hayward, CA) ; Shinde; Ranjit Indrajit;
(Yelahanka, IN) ; Cho; Seon-Mee; (Santa Clara,
CA) ; Truong; Douglas D.; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kudela; Jozef
Tanaka; Tsutomu
Sorensen; Carl A.
Anwar; Suhail
White; John M.
Shinde; Ranjit Indrajit
Cho; Seon-Mee
Truong; Douglas D. |
San Jose
Santa Clara
Morgan Hill
San Jose
Hayward
Yelahanka
Santa Clara
San Jose |
CA
CA
CA
CA
CA
CA
CA |
US
US
US
US
US
IN
US
US |
|
|
Family ID: |
49002081 |
Appl. No.: |
13/507383 |
Filed: |
June 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13282469 |
Oct 27, 2011 |
|
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13507383 |
|
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|
61499205 |
Jun 21, 2011 |
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Current U.S.
Class: |
313/231.31 ;
29/854; 333/237 |
Current CPC
Class: |
H01Q 13/22 20130101;
H05H 2001/463 20130101; H01P 11/003 20130101; H01P 3/085 20130101;
H01J 37/3222 20130101; H05H 1/24 20130101; H05H 1/46 20130101; Y10T
29/49169 20150115; H01J 37/32577 20130101; H01P 3/00 20130101; H01P
3/06 20130101; H01J 37/32082 20130101 |
Class at
Publication: |
313/231.31 ;
333/237; 29/854 |
International
Class: |
H01P 3/00 20060101
H01P003/00; H01P 11/00 20060101 H01P011/00; H05H 1/24 20060101
H05H001/24 |
Claims
1. A transmission line RF applicator for coupling electrical power
to a plasma outside the RF applicator, comprising: an outer
conductor having a main portion extending between first and second
end portions; and an inner conductor having a main portion
extending between first and second end portions, wherein the main
portion of the inner conductor is positioned within, and spaced
away from, the main portion of the outer conductor; wherein the
main portion of the outer conductor includes: (i) an inner surface
facing the main portion of the inner conductor, (ii) an outer
surface, and (iii) a plurality of apertures that extend between the
inner surface of the outer conductor and the outer surface of the
outer conductor.
2. The applicator of claim 1, further comprising: a dielectric
cover; wherein the main portion of each of said inner and outer
conductors is positioned within the dielectric cover; and wherein
the dielectric cover provides a gas seal around the main portion of
each of said conductors such that gas cannot flow between the
exterior of the dielectric cover and the main portion of either of
said conductors.
3. The applicator of claim 1, wherein: the outer conductor has a
tubular shape; and the inner conductor and the outer conductor are
positioned coaxially.
4. A plasma chamber comprising: a vacuum enclosure that encloses an
interior of the plasma chamber; a dielectric cover having a main
portion extending between first and second end portions, wherein
the main portion of the dielectric cover is positioned within said
interior of the plasma chamber; an outer conductor having a main
portion extending between first and second end portions, wherein
the main portion of the outer conductor is positioned within the
main portion of the dielectric cover; an inner conductor having a
main portion extending between first and second end portions,
wherein the main portion of the inner conductor is positioned
within, and spaced away from, the main portion of the outer
conductor; and first and second sealing apparatuses that
respectively abut the first and second end portions of the
dielectric cover such that the first and second sealing
apparatuses, the dielectric cover and the vacuum enclosure in
combination prevent fluid communication between the main portion of
the outer conductor and the interior of the plasma chamber; wherein
the main portion of the outer conductor includes: (i) an inner
surface facing the main portion of the inner conductor, (ii) an
outer surface facing an inner surface of the main portion of the
dielectric cover, and (iii) two or more apertures that extend
between the inner surface of the outer conductor and the outer
surface of the outer conductor.
5. The plasma chamber of claim 4, wherein: the space between the
main portion of the inner conductor and the main portion of the
outer conductor is open to ambient atmosphere such that said space
remains at ambient atmospheric pressure regardless of the pressure
within the interior of the plasma chamber.
6. The plasma chamber of claim 4, wherein: the space between the
main portion of the inner conductor and the main portion of the
outer conductor is at least partly occupied by a gas; and the first
and second sealing apparatuses provide a gas-tight seal between
said space and the interior of the plasma chamber so as to enable a
pressure differential between said space and the interior of the
plasma chamber.
7. The plasma chamber of claim 4, wherein: the first sealing
apparatus extends between the first end portion of the dielectric
cover and the vacuum enclosure.
8. The plasma chamber of claim 4, wherein: the second sealing
apparatus is positioned within the interior of the plasma chamber
and does not abut the vacuum enclosure.
9. The plasma chamber of claim 4, further comprising: an RF power
source that is connected to produce an RF voltage between the inner
conductor and the outer conductor.
10. The plasma chamber of claim 4, further comprising: a first RF
power source that is connected to produce a first RF voltage
between the first end portion of the inner conductor and the first
end portion of the outer conductor; and a second RF power source
that is connected to produce a second RF voltage between the second
end portion of the inner conductor and the second end portion of
the outer conductor.
11. The plasma chamber of claim 4, further comprising: an RF power
source that is connected to produce an RF voltage between the first
end portion of the inner conductor and the first end portion of the
outer conductor; and a termination impedance that is connected
between the second end portion of the inner conductor and the
second end portion of the outer conductor.
12. The plasma chamber of claim 4, further comprising: an RF power
source having an RF power output that is connected between the
inner conductor and the outer conductor; wherein the RF power
output has a wavelength that is shorter than the longest dimension
of the main portion of the inner conductor and is shorter than the
longest dimension of the main portion of the outer conductor.
13. The plasma chamber of claim 4, wherein said two or more
apertures include: a plurality of apertures at different
longitudinal positions on the outer conductor; wherein adjacent
ones of said plurality of apertures at successive longitudinal
positions on the outer conductor are offset along the
circumferential dimension of the outer conductor.
14. The plasma chamber of claim 4, wherein said two or more
apertures include: a first plurality of apertures in a first
sub-portion of the main portion of the outer conductor and a second
plurality of apertures in a distinct second sub-portion of the main
portion of the outer conductor; wherein: the first sub-portion
extends from the first end portion of the outer conductor to the
second sub-portion; the second sub-portion extends from the first
sub-portion to the center of the outer conductor; and the fraction
of the surface area of the second portion that is occupied by the
second plurality of apertures is greater than the fraction of the
surface area of the first portion that is occupied by the first
plurality of apertures.
15. The plasma chamber of claim 4, wherein said two or more
apertures include: a first plurality of apertures in a first
sub-portion of the main portion of the outer conductor and a second
plurality of apertures in a distinct second sub-portion of the main
portion of the outer conductor; wherein: the first sub-portion
extends from the first end portion of the outer conductor to the
second sub-portion; the second sub-portion extends from the first
sub-portion to the center of the outer conductor; and the average
area of the apertures in the second sub-portion is greater than the
average area of the apertures in the first sub-portion.
16. The plasma chamber of claim 4, wherein said two or more
apertures include: a first plurality of apertures in a first
sub-portion of the main portion of the outer conductor and a second
plurality of apertures in a distinct second sub-portion of the main
portion of the outer conductor; wherein: the first sub-portion
extends from the first end portion of the outer conductor to the
second sub-portion; the second sub-portion extends from the first
sub-portion to the center of the outer conductor; and the average
spacing between adjacent apertures in the second sub-portion is
smaller than the average spacing between adjacent apertures in the
first sub-portion.
17. The plasma chamber of claim 4, wherein said two or more
apertures include: a first plurality of apertures in a first
sub-portion of the main portion of the outer conductor and a second
plurality of apertures in a distinct second sub-portion of the main
portion of the outer conductor; wherein: the first sub-portion
extends from the first end portion of the outer conductor to the
second sub-portion; the second sub-portion extends from the first
sub-portion to the center of the outer conductor; and the average
of said angles of the apertures in the second sub-portion is
smaller than the average of said angles of the apertures in the
first sub-portion.
18. The plasma chamber of claim 4, wherein said two or more
apertures include: a plurality of apertures at successive positions
progressing from a first position to a second position on the main
portion of the outer conductor; wherein: the first position is
between the second position and the first end portion of the outer
conductor; wherein the second position is between the first
position and the center of the outer conductor; and each respective
aperture at said respective positions progressing from the first
position to the second position has a monotonically increasing
area.
19. The plasma chamber of claim 4, wherein said two or more
apertures include: a plurality of apertures at successive positions
progressing from a first position to a second position on the main
portion of the outer conductor; wherein: the first position is
between the second position and the first end portion of the outer
conductor; the second position is between the first position and
the center of the outer conductor; and each respective aperture at
said respective positions progressing from the first position to
the second position has a monotonically decreasing spacing between
adjacent apertures.
20. The plasma chamber of claim 4, wherein said two or more
apertures include: a plurality of apertures at successive positions
progressing from a first position to a second position on the main
portion of the outer conductor; wherein: the first position is
between the second position and the first end portion of the outer
conductor; the second position is between the first position and
the center of the outer conductor; and each respective aperture at
said respective positions progressing from the first position to
the second position has a long axis at a monotonically decreasing
angle relative to the circumferential dimension of the outer
conductor.
21-22. (canceled)
23. A method of coupling electrical power to a plasma, comprising
the steps of: providing an outer conductor having a main portion
extending between first and second end portions; and providing an
inner conductor having a main portion extending between first and
second end portions, wherein the main portion of the inner
conductor is positioned within, and spaced away from, the main
portion of the outer conductor; wherein the main portion of the
outer conductor includes: (i) an inner surface facing the main
portion of the inner conductor, (ii) an outer surface, and (iii) a
plurality of apertures that extend between the inner surface of the
outer conductor and the outer surface of the outer conductor.
24. The method of claim 23, further comprising the steps of:
providing a vacuum enclosure that encloses an interior of a plasma
chamber; providing a dielectric cover having a main portion
extending between first and second end portions, wherein the main
portion of the dielectric cover is positioned within said interior
of the plasma chamber, and wherein the main portion of the outer
conductor is positioned within the main portion of the dielectric
cover such that the outer surface of outer conductor faces an inner
surface of the main portion of the dielectric cover; and providing
first and second sealing apparatuses that respectively abut the
first and second end portions of the dielectric cover such that the
first and second sealing apparatuses, the dielectric cover and the
vacuum enclosure in combination prevent fluid communication between
the main portion of the outer conductor and the interior of the
plasma chamber.
25. A method of coupling electrical power to a plasma, comprising
the steps of: providing a first outer conductor having a main
portion extending between first and second end portions; providing
a second outer conductor having a main portion extending between
first and second end portions; and providing an inner conductor
having a main portion extending between first and second end
portions, wherein the main portion of the inner conductor is
positioned between and spaced away from the main portion of the
first outer conductor and the main portion of the second outer
conductor; wherein the main portion of each respective outer
conductor includes: (i) an inner surface facing the main portion of
the inner conductor, (ii) an outer surface, and (iii) a plurality
of apertures that extend between the inner surface of said
respective outer conductor and the outer surface of said respective
outer conductor.
26-35. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This patent application is a continuation-in-part of U.S.
patent application Ser. No. 13/282,469 filed Jan. 18, 2012 and
claims the benefit of U.S. Provisional Application No. 61/499,205
filed Jun. 21, 2011.
TECHNICAL FIELD
[0002] The invention relates generally to RF applicator apparatus
and methods useful for coupling RF power to a plasma discharge in a
plasma chamber for fabricating electronic devices such as
semiconductors, displays and solar cells. The invention relates
more specifically to such an RF applicator comprising an inner
conductor and one or two outer conductors, wherein each outer
conductor has apertures from which the RF applicator can radiate RF
energy to a plasma in a plasma chamber.
BACKGROUND ART
[0003] Plasma chambers commonly are used to perform processes for
fabricating electronic devices such as semiconductors, displays and
solar cells. Such plasma fabrication processes include chemical
vapor deposition of semiconductor, conductor or dielectric layers
on the surface of a workpiece or etching of selected portions of
such layers on the workpiece surface.
[0004] A plasma commonly is sustained in a plasma chamber by
coupling RF power from an RF applicator to a gas or plasma within
the chamber. The RF power excites the gas to a plasma state or
provides the RF power necessary to sustain the plasma. Two broad
categories of coupling techniques are an electrode that
capacitively couples RF power to the plasma or an antenna that
radiates electromagnetic radiation into the plasma.
[0005] One conventional type of antenna is an inductive coupler,
also called an inductively coupled antenna, in which RF power is
coupled to the plasma primarily by means of the magnetic field
produced by the antenna. A shortcoming of an inductive coupler is
that it generally cannot be operated at an RF frequency whose
wavelength is less than the diameter of the inductive coupler. The
inability to operate at a high RF frequency is a serious
shortcoming in certain plasma chemistries.
[0006] Another conventional type of antenna is a hollow waveguide
having slots in one waveguide wall through which RF power is
radiated from the interior of the hollow waveguide to the plasma. A
shortcoming of a hollow waveguide is that it cannot operate below a
cutoff frequency, hence its width along one transverse axis must be
at least one-half the wavelength of a signal propagating within the
waveguide at the power source frequency.
[0007] As a result of this width requirement, slotted hollow
waveguide antennas typically have been used outside a dielectric
window of a plasma chamber rather than inside a plasma chamber.
[0008] Another conventional type of antenna is a linear conductor
surrounded by a cylindrical dielectric, with the combination being
positioned within a plasma chamber so that it is surrounded by the
plasma. One or both ends of the conductor are connected to receive
power from a UHF or microwave power source. Power is coupled from
the antenna to the plasma by means of an electromagnetic wave at
the boundary between the plasma and the dielectric. A shortcoming
of this type of antenna is that the power radiated by the antenna
progressively decreases with distance from the end of the antenna
that is connected to the power source. Even if both ends of the
antenna are connected to a power source, the radiated power near
the center of the antenna will be lower than the power near the
ends, thereby degrading spatial uniformity of the plasma. The
non-uniformity increases with the length of the antenna, hence this
type of antenna is less desirable for large plasma chambers.
SUMMARY OF THE INVENTION
[0009] The invention is a transmission line RF applicator apparatus
and method useful for coupling RF power to a plasma in a plasma
chamber. The invention comprises an inner conductor and one or two
outer conductors. The main portion of each of the one or two outer
conductors includes a plurality of apertures that extend between an
inner surface and an outer surface of the outer conductor.
[0010] In operation, when the output of an RF power source is
connected between the inner conductor and the one or two outer
conductors, the RF applicator radiates RF energy from the apertures
in the one or two outer conductors. A single RF power source can be
connected to the inner and outer conductors or, more preferably,
two RF power supplies can be connected respectively to opposite
ends of the RF applicator.
[0011] Another aspect of the invention is a plasma chamber that
includes the aforesaid transmission line RF applicator in
combination with a dielectric cover and first and second sealing
apparatuses. The plasma chamber comprises a vacuum enclosure that
encloses an interior of the plasma chamber. A main portion of the
dielectric cover is positioned within the interior of the plasma
chamber. The main portion of the aforesaid one or two outer
conductors is positioned within the main portion of the dielectric
cover. The first and second sealing apparatuses respectively abut
first and second end portions of the dielectric cover such that the
first and second sealing apparatuses, the dielectric cover and the
vacuum enclosure in combination prevent fluid communication between
the main portion of the outer conductor and the interior of the
plasma chamber.
[0012] Preventing such fluid communication is advantageous to
prevent the formation within the apertures of a gas discharge that
would electrically short-circuit the apertures, thereby preventing
the RF applicator from radiating RF power through the apertures.
Furthermore, if any portion of the space between the inner and
outer conductors is occupied by a gas, an additional advantage of
preventing such fluid communication is that, during operation of
the plasma chamber, it enables the space to remain at a much higher
pressure than the vacuum within the plasma chamber. Maintaining the
space at a higher pressure, such as atmospheric pressure, helps
prevent gas discharge between the inner and outer conductors.
[0013] In a first aspect or embodiment of the invention, the inner
conductor is positioned within the outer conductor, and there is no
requirement for more than one outer conductor. In a second aspect
or embodiment of the invention requiring two outer conductors, the
inner conductor is positioned between the two outer conductors.
[0014] In operation, the amount of power radiated from any portion
of the RF applicator increases with the number and size of the
apertures in that portion and with the respective angles at which
the apertures are oriented relative to the longitudinal dimension
of the RF applicator.
[0015] Therefore, one advantage of the invention is that the RF
applicator can be as long as desired by employing apertures that
are sufficiently small and widely spaced to avoid the power
propagating within the RF applicator from dropping to zero at
longitudinal positions farthest from where the one or two outer
conductors are connected to an RF power source.
[0016] A second advantage of the invention is that, unlike a hollow
waveguide, the RF applicator does not have a cutoff frequency,
hence its transverse width is not required to be greater than
one-half wavelength as would be required in a hollow waveguide.
[0017] A third advantage of the invention is that, unlike an
inductive coupler, the RF applicator can be operated at an RF
frequency whose wavelength is shorter than the longest dimension of
the portion of the RF applicator that radiates RF. In other words,
the output of the RF power source can have a wavelength that is
shorter than the longest dimension of the main portion of the inner
conductor and is shorter than the longest dimension of the main
portion of the outer conductor.
[0018] A further invention that is useful both with the aforesaid
RF applicator and with other RF applicators having at least two
distinct conductors is that the spatial uniformity of radiated
power or the spatial uniformity of the plasma can be optimized by
altering the relative sizes, spacing or orientations of apertures
in different portions of the one or two outer conductors.
[0019] A further invention that is useful both with the aforesaid
RF applicator and with other RF applicators having at least two
distinct conductors is that the efficiency of radiation of RF power
can be improved by providing an offset in a transverse or
circumferential direction between apertures at successive
longitudinal positions.
[0020] Within this patent application, we use the term RF to
broadly include the microwave frequency range and all frequencies
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a longitudinal sectional view of a plasma chamber
including a two- conductor RF applicator according to the
invention, with the connection of the RF applicator to two RF power
sources shown schematically.
[0022] FIG. 2 is a longitudinal sectional view of an embodiment
identical to FIG. 1 except having only one RF power source.
[0023] FIG. 3 is a sectional view of a detail of the first and
second ends of the RF applicator of FIGS. 1 and 2.
[0024] FIG. 4 is a transverse sectional view of the second end of
the RF applicator of FIGS. 1 and 2 where it passes through a wall
of the vacuum enclosure.
[0025] FIG. 5 is a side view of the outer conductor of FIGS.
1-4.
[0026] FIG. 6 is a transverse sectional view of the outer conductor
of FIG. 5.
[0027] FIG. 7 is a transverse sectional view of an alternative RF
applicator whose outer conductor has an elliptical cross
section.
[0028] FIG. 8 is a transverse sectional view of an alternative RF
applicator whose inner and outer conductors have rectangular cross
sections.
[0029] FIG. 9 is a longitudinal sectional view of a variation of
the embodiment of FIG. 2 having alternative first and second
sealing apparatuses.
[0030] FIG. 10 is a cross sectional detail of a portion of the
outer conductor taken through the section lines shown in FIG. 1 or
FIG. 2.
[0031] FIGS. 11 and 12 are alternative embodiments of the portion
of the outer conductor shown in FIG. 10.
[0032] FIG. 13 is a cross sectional detail of a portion of the
outer conductor taken through the section lines shown in FIG.
2.
[0033] FIGS. 14 and 15 are a side view and a perspective view of an
alternative embodiment of the outer conductor having a 90-degree
azimuthal offset between successive apertures.
[0034] FIGS. 16 and 17 are sectional views of the outer conductor
of FIG. 14.
[0035] FIGS. 18 and 19 are a side view and a perspective view of an
alternative embodiment of the outer conductor having a 60-degree
azimuthal offset between successive apertures.
[0036] FIGS. 20-22 are sectional views of the outer conductor of
FIG. 18.
[0037] FIG. 23 is a longitudinal sectional view of a plasma chamber
including a three-conductor RF applicator according to the
invention, with the connection of the RF applicator to two RF power
sources shown schematically.
[0038] FIG. 24 is a transverse sectional view of the RF applicator
of FIG. 23.
[0039] FIG. 25 is a transverse sectional view of a modification of
the RF applicator of FIG. 23 wherein each outer conductor has an
arcuate cross section.
BEST MODE FOR CARRYING OUT THE INVENTION
1. Two-Conductor RF Applicator
[0040] FIGS. 1-22 illustrate various embodiments of a two-conductor
transmission line RF applicator 10 according to the first aspect or
first embodiment of the invention.
[0041] The RF applicator 10 includes an inner conductor 14 and an
outer conductor 20. The outer conductor 20 has a main portion 21
extending between first and second end portions 24,25. Similarly,
the inner conductor 14 has a main portion 15 extending between
first and second end portions 16,17. The main portion 15 of the
inner conductor is positioned within, and spaced away from, the
main portion 21 of the outer conductor 20.
[0042] We refer to the RF applicator 10 as having opposite first
and second ends 12,13, such that the first end 12 of the RF
applicator is adjacent the respective first end portions 16, 24 of
the inner and outer conductors, and the second end 13 of the RF
applicator is adjacent the respective second end portions 17,25 of
the inner and outer conductors.
[0043] The main portion 21 of the outer conductor 20 includes a
plurality of apertures 30 that extend between inner and outer
surfaces 22,23 of the main portion of the outer conductor. The
inner surface 22 faces the main portion 15 of the inner conductor.
In embodiments that include a dielectric cover 40 as explained
below, the outer surface 23 of the main portion of the outer
conductor faces the inner surface 44 of the main portion 41 of the
dielectric cover.
[0044] In operation, when the output of an RF power source 70,74 is
connected between the inner conductor 14 and the outer conductor
20, an RF electromagnetic wave propagates through the space 18
between the respective main portions 15,21 of the inner and outer
conductors. A portion of the RF power in this electromagnetic wave
radiates from the apertures 30, thereby radiating RF power outside
the RF applicator.
[0045] If the RF applicator is within the vacuum enclosure 60 of a
plasma chamber as shown in FIGS. 1-4, the RF power radiated by the
RF applicator will be absorbed by the gases and plasma within the
plasma chamber and thereby excite the gases to a plasma state or
sustain an existing plasma.
[0046] The invention is especially advantageous for use in a plasma
chamber that processes two workpieces 62 simultaneously. In that
case, an RF applicator 10 according to the invention can be
positioned between the two workpieces 62 within the vacuum
enclosure 60 of a plasma chamber as shown in FIGS. 1 and 2 so as to
provide equal plasma densities adjacent the two workpieces.
Optionally, an array of multiple RF applicators 10 can be
positioned within the vacuum enclosure of the plasma chamber so as
to distribute the RF power over a wider area than a single RF
applicator. For example, the multiple RF applicators 10 can be
spaced apart within a geometric plane that is equidistant between
the two workpieces.
[0047] The RF applicator preferably includes a dielectric cover 40
and first and second sealing apparatuses 52,53 to prevent plasma
from entering the apertures 30. This is explained in the subsequent
section of this patent specification entitled "3. Dielectric Cover
and Dielectric Between Conductors".
[0048] If only one RF power source 70 is connected to the RF
applicator as shown in FIG. 2, the electromagnetic wave propagating
within the RF applicator will have a standing wave spatial
distribution pattern in which the electric field will have
alternating maxima and minima every one-fourth wavelength along the
length of the RF applicator. In this standing wave pattern, the
axial component of the electric field has a maximum at points where
the radial component of the electric field has a minimum, and vice
versa. Any apertures 30 located near a maximum of the axial
electrical field standing wave pattern will radiate much more power
than any apertures of the same size and orientation located near a
minimum of the axial electrical field standing wave pattern.
[0049] It would be possible to position the apertures 20 only at
locations of successive maxima of the axial electrical field
standing wave pattern, which would occur at half-wavelength
intervals along the longitudinal dimension L of the outer
conductor. However, the locations of the maxima are difficult to
predict because the standing wave pattern shifts as a function of
operating conditions in the plasma chamber. Therefore, if only one
RF power source 70 is connected to the RF applicator, it is
preferable to space the apertures less than one-fourth wavelength
apart along the longitudinal dimension of the outer conductor, in
which case there is no need to predict the locations of the
standing wave maxima.
[0050] A key difference between the invention and conventional
designs that employ a slotted hollow waveguide RF applicator is
that the invention has distinct inner and outer RF-powered
conductors 14, 20 that can be connected to receive an RF voltage
from an RF power source 70. (In other words, an RF power source can
be connected to produce an RF voltage between the inner conductor
14 and the outer conductor 20.) In contrast, the waveguide of a
hollow waveguide RF applicator is not RF-powered, but merely
functions as an electrically conductive boundary to confine a wave
propagating through the dielectric that the hollow waveguide
surrounds. It is well known that a hollow waveguide has a cutoff
frequency below which no wave will propagate, which requires its
transverse width to exceed a certain size. Reducing the transverse
width of the RF applicator is beneficial to reduce the fraction of
the reagents in the plasma chamber that are consumed by surface
reactions adjacent the surface of the RF applicator. A valuable
advantage of the invention over slotted hollow waveguide RF
applicators is that the invention does not have a cutoff frequency
or a required minimum dimension.
[0051] The invention does not require the inner and outer
conductors 14,20 to have any specific shapes. In FIGS. 4-6, the
main portion 15 of the inner conductor 14 and the main portion 21
of the outer conductor 20 each have a circular cross section. FIG.
7 illustrates an alternative embodiment of an RF applicator 10 in
which the main portion 21 of the outer conductor 20 has an
elliptical cross section. FIG. 8 illustrates an alternative
embodiment of an RF applicator 10 in which the respective main
portions 15,21 of the inner and outer conductors 14,20 each have
rectangular cross sections.
[0052] The inner conductor need not have the same shape as the
outer conductor. For example, an RF applicator can have an inner
conductor 14 that is cylindrical as in FIG. 7 in combination with
an outer conductor 20 that has a rectangular cross section as in
FIG. 8.
[0053] In all of the illustrated embodiments, the inner and outer
conductors are positioned coaxially and are straight and tubular in
shape. However, this is not a requirement of the invention. For
example, the inner and outer conductors can have a curved,
serpentine or zig-zag shape.
2. Connections to RF Power Source
[0054] The details of electrical connections from one or two RF
power sources 70,74 to the RF applicator 10 now will be
described.
[0055] In operation, a first RF power source 70 is connected to
produce a first RF voltage between the inner conductor 14 and the
outer conductor 20. Preferably but optionally, a second RF power
source 74 is connected to produce a second RF voltage between the
inner conductor 14 and the outer conductor 20.
[0056] If both RF power sources are used, preferably the RF outputs
of the first and second RF power sources 70,74 are respectively
connected to the first and second ends 12,13, respectively, of the
RF applicator as shown in FIG. 1. If only the first RF power source
is used as shown in FIG. 2, its RF output can be connected to any
locations on the inner and outer conductors 14, 20.
[0057] More specifically, if both RF power sources are used as in
FIG. 1, preferably the first RF power source 70 is connected to
produce a first RF voltage between the first end portion 16 of the
inner conductor 14 and the first end portion 24 of the outer
conductor 20. Likewise, preferably the second RF power source 74 is
connected to produce a second RF voltage between the second end
portion 17 of the inner conductor 14 and the second end portion 25
of the outer conductor.
[0058] Alternatively, if only the first RF power source is used as
in FIG. 2, its output can be connected to produce an RF voltage
between any location on the inner conductor 14 and any location on
the outer conductor 20. Preferably, the first RF power source is
connect to the first end 12 of the RF applicator, and a termination
impedance 79 is connected to the second end 13 of the RF
applicator. Specifically, the first RF power source 70 preferably
is connected to produce an RF voltage between the first end portion
16 of the inner conductor 14 and the first end portion 24 of the
outer conductor 20. The termination impedance 79 preferably is
connected between the second end portion 17 of the inner conductor
14 and the second end portion 25 of the outer conductor 20.
[0059] The termination impedance 79 can be any electrical
impedance. For example, the termination impedance 79 can be an
electrical short circuit or a conventional tuning plunger, and
optionally it can be movable along the longitudinal dimension L of
the inner and outer conductors 14, 20.
[0060] In operation, the RF power supplied by the first, and
optionally second, RF power sources 70, 74 produces an
electromagnetic field in the space 18 between the respective main
portions 15, 21 of the inner and outer conductors 14, 20 that
propagates as an RF electromagnetic wave along the length of such
space 18 between the first and second ends 12, 13 of the RF
applicator.
[0061] If only one RF power source 70 is connected to the inner and
outer conductors as in FIG. 2, the wave propagating within the RF
applicator will be a standing wave.
[0062] Alternatively, if two independent (i.e., not phase-coherent)
RF power sources 70, 74 are connected to opposite end portions of
the inner and outer conductors as in FIG. 1, the wave propagating
within the RF applicator will be a traveling wave. In the latter
case, each power source preferably includes at its output a
conventional RF isolator 78 for the purpose of preventing the wave
propagating from one RF power source to the opposite RF power
source from being reflected back into the RF applicator, thereby
preventing the creation of a standing wave within the RF
applicator.
[0063] All outputs of the power sources 70,74 are shown in FIGS. 1
and 2 as floating, i.e., as not connected to electrical ground.
Alternatively, one of the outputs from each power source can be
electrically grounded.
[0064] When we describe an output of an RF power source 70,74 as
connected to any of the conductors 14,20 of the RF applicator, the
connection can be through intermediate components, such as an RF
transformer, an impedance matching network, or a hollow waveguide
transmission line connected between an RF power source and one or
more conductors of the RF applicator. The only requirement of the
invention is that the connection of the RF power source 70 or 74 to
the RF applicator--with or without intermediate components--is
configured such that the RF power source produces an RF voltage
between the inner conductor 14 and the outer conductor 20.
[0065] To accommodate thermal expansion of the inner and outer
conductors 14,20, the aforesaid electrical connection of RF power
to the inner and outer conductors optionally includes conventional
sliding finger contacts.
[0066] If the RF power signal produced by the RF power source 70,74
is in the microwave frequency range, a hollow waveguide can be an
efficient means for connecting the output of the RF power source to
the inner and outer conductors. In general, the hollow waveguide is
coupled to the output of the RF power source so that the RF power
produced by the RF power source propagates as an electromagnetic
wave through the interior of the waveguide. The hollow waveguide is
coupled to the respective first end portions 15,21 of the inner and
outer conductors so that the RF wave in the waveguide produces an
RF voltage between the inner conductor 14 and each outer conductor
20 of the RF applicator. Any conventional coupler for extracting an
RF voltage from a hollow waveguide can be used.
[0067] It is important to emphasize that the use of a hollow
waveguide to connect the output of an RF power source to the
respective first end portions 15, 21 of the inner and outer
conductors does not imply that the RF applicator 10 is similar to a
hollow waveguide. As stated at the end of the preceding section of
this patent specification entitled "1. Two-Conductor RF
applicator", our RF applicator 10 has a plurality of RF-powered
conductors 14, 20. In contrast, the waveguide of a hollow waveguide
RF applicator is not RF-powered, but merely functions as an
electrically conductive boundary to confine a wave propagating
through the dielectric that the hollow waveguide surrounds. This
difference is responsible for an important advantage of the
invention, which is that it has no cutoff frequency and no required
minimum dimension.
[0068] As stated above, an array of multiple RF applicators 10
optionally can be positioned within the vacuum enclosure of the
plasma chamber. Each respective RF applicator can be connected to a
distinct respective first power source 70 and, optionally, a
distinct respective second power source 74. Alternatively, multiple
RF applicators can be connected in parallel to the same power
source. Alternatively, multiple RF applicators can be connected in
series to a single power source 70 or in series between first and
second power sources 70, 74. If multiple RF applicators are
connected in series, then at the junction between any two of the RF
applicators, each of the two RF applicators functions as a
termination impedance for the other RF applicator.
3. Dielectric Cover and Dielectric Between Conductors
[0069] If the apertures 30 have a transverse width that exceeds a
certain value (which is a function of chamber pressure and process
gas composition), a gas discharge can form within the apertures if
gas within the interior of the plasma chamber is permitted to enter
the apertures. Such gas discharge would electrically short-circuit
the apertures, thereby preventing the RF applicator from radiating
RF power through the apertures.
[0070] To permit the use of larger apertures without risk of gas
discharge within the apertures, the RF applicator 10 preferably
includes a dielectric cover 40 and first and second sealing
apparatuses 52,53.
[0071] The plasma chamber includes a vacuum enclosure 60 that
encloses the interior 61 of the plasma chamber. The vacuum
enclosure 60 includes one or more walls that collectively provide
an air-tight enclosure that enables a vacuum to be maintained in
the interior 61 if a vacuum pump is coupled to the interior. The
dielectric cover includes a main portion 41 that extends between
first and second end portions 42,43. The main portion of the
dielectric cover is positioned within said interior 61 of the
plasma chamber. The main portion 21 of the outer conductor 20 is
positioned within the main portion 41 of the dielectric cover
40.
[0072] The first sealing apparatus 52 abuts the first end portion
42 of the dielectric cover 40, and the second sealing apparatus 53
abuts the second end portion 43 of the dielectric cover. The first
and second sealing apparatuses, the dielectric cover and the vacuum
enclosure 60 in combination prevent fluid communication between the
main portion of the outer conductor and the interior 61 of the
plasma chamber. Consequently, the dielectric cover 40 prevents gas
(or plasma) within the plasma chamber from entering the apertures
30.
[0073] Typically it does not matter whether the first and second
sealing apparatuses 52,53 are dielectric or conductive because they
typically are not electrically coupled to the inner conductor 14 or
outer conductor 20.
[0074] In the embodiments illustrated in FIGS. 1-4, the first and
second end portions of the dielectric cover 40 either abut or
extend through opposite sides of the vacuum enclosure 60 of the
plasma chamber. These embodiments illustrate that each of the first
and second sealing apparatuses 52,53 optionally can be merely a
conventional O-ring. The first sealing apparatus 52 is an O-ring
that extends between the first end portion 42 of the dielectric
cover and the vacuum enclosure 60, and the second sealing apparatus
53 is an O-ring that extends between the second end portion 43 of
the dielectric cover and the vacuum enclosure 60. Each sealing
apparatus 52,53--i.e., each O-ring--provides a hermetic seal
between the dielectric cover 40 and the vacuum enclosure 60.
Consequently, the two O-rings, the dielectric cover and the vacuum
enclosure in combination prevent fluid communication between the
main portion of the outer conductor and the interior 61 of the
plasma chamber.
[0075] An advantage of the O-rings 52,53 illustrated in FIGS. 1-4
is that they can accommodate thermal expansion of the dielectric
cover 40 by permitting the dielectric cover to move (along the
longitudinal dimension L of the dielectric cover) relative to the
vacuum enclosure 60 while maintaining the hermetic seal described
in the preceding paragraph.
[0076] Depending on the types of materials of which the inner and
outer conductors 14,20 and the dielectric cover 40 are composed,
the inner and outer conductors may have a higher thermal expansion
coefficient than the dielectric cover. If so, the outer conductor
preferably is mounted so that it is free to slide longitudinally
within the dielectric cover, thereby accommodating thermal
expansion of the outer conductor while minimizing thermal stress in
the dielectric cover.
[0077] FIG. 9 illustrates two alternative embodiments of the
sealing apparatuses 52,53. The first sealing apparatus 52 includes
a collar 54 and two O-rings 55,56. The first O-ring 55 provides a
hermetic seal between the collar 54 and the first end portion 42 of
the dielectric cover 40. The second O-ring 56 provides a hermetic
seal between the collar 54 and the plasma chamber's vacuum
enclosure 60. The first sealing apparatus 52--i.e., the collar 54
and the two O-rings 55,56 in combination--thereby provides a
hermetic seal between the dielectric cover 40 and the vacuum
enclosure 60.
[0078] FIG. 9 also illustrates an alternative design for the second
end 13 of the RF applicator 10. Specifically, the termination
impedance 79 is positioned within the dielectric cover 40, thereby
eliminating any need for the second end portion 17 of the inner
conductor 14 and the second end portion 25 of the outer conductor
20 to pass through the vacuum enclosure of the vacuum chamber (as
otherwise would be required to connect to an externally located
termination impedance 79 as in FIG. 2 or an externally located
power source 54 as in FIG. 1). This eliminates any need for the
second end portion 43 of the dielectric cover to abut or pass
through the vacuum enclosure 60 of the plasma chamber.
[0079] As described above, the termination impedance 79 can be any
electrical impedance. For example, the termination impedance 79 can
simply be a conductor (i.e., an electrical short circuit) connected
between the second end portion of the inner conductor 14 and the
second end portion of the outer conductor 20 as shown in FIG. 9.
Alternatively, the second end portions of the inner and outer
conductors can be left open, so that the termination impedance
would be an open circuit or the parasitic impedance between the
second end portions of the inner and outer conductors.
[0080] In the alternative design of FIG. 24, because the second end
portion 43 of the dielectric cover does not abut or pass through
the vacuum enclosure 60, the second sealing apparatus 53 can be
spaced away from the vacuum enclosure 60. In the example of FIG.
24, the second sealing apparatus 53 includes a dielectric end cap
58 and an O-ring 59. The dielectric end cap 58 overlies the opening
at the second end portion 43 of the dielectric cover, and the
O-ring 59 provides a hermetic seal between the dielectric end cap
58 and the second end portion of the dielectric cover.
[0081] In a variation of this design (not shown), the dielectric
end cap 58 can be integral and contiguous with the second end
portion 43 of the dielectric cover, thereby providing the hermetic
seal described in the preceding paragraph without need for the
O-ring 59.
[0082] The space 18 between the main portion 15 of the inner
conductor 14 and the main portion 21 of the outer conductor 20 can
be occupied by any type of dielectric, which can be any combination
of gas, liquid or solid dielectrics. To maximize the efficiency of
the RF applicator, the dielectric occupying the space 18 preferably
is a material having a low absorption of energy at the operating
frequencies of the RF power sources. For example, deionized water
would be a suitable dielectric at certain RF frequencies, but it
would be a bad choice if the RF power source were operated at 2.4
GHz because water absorbs radiation at that frequency.
[0083] Air typically is a suitable dielectric for the space 18
between the main portion 15 of the inner conductor 14 and the main
portion 21 of the outer conductor 20. Therefore, the space 18 can
simply be open to ambient atmosphere, as shown in FIGS. 1-3, 9 and
23. In that case, the space 18 remains at ambient atmospheric
pressure regardless of the pressure (i.e., vacuum) within the
interior of the plasma chamber.
[0084] The dielectric occupying the space 18 optionally can be a
fluid that is pumped through the space 18 in order to absorb heat
from the inner and outer conductors 14,20. The fluid can be a
liquid or a gas such as air or nitrogen. After flowing through the
space 18, the fluid can be discharged outside the plasma chamber or
recirculated through a heat exchanger, thereby cooling the RF
applicator. Such cooling is beneficial because the dielectric cover
40 is heated by the plasma in the plasma chamber, and heat flows
from the dielectric cover to the outer conductor 20. In addition,
the inner conductor 14 is heated by resistive heating caused by RF
current flow through the inner conductor.
[0085] The inner conductor 14 can be solid or hollow. If it is
hollow, additional cooling of the inner conductor can be provided
by pumping a coolant fluid such as water through its hollow
interior. There is essentially no RF field in the interior of the
inner conductor, so the electrical properties of this coolant fluid
are unimportant.
[0086] If the space 18 is occupied by a fluid as just described, it
may be desirable to stabilize the position of the inner conductor
14 relative to the outer conductor 20 by mechanically connecting
one or more support members (not shown) between the inner conductor
14 and the outer conductor 20. The support members preferably are a
dielectric material such as PTFE (polytetrafluoroethene).
Alternatively, the support members can be electrically conductive
if the support members have a small transverse width, thereby
minimizing the disruption of the electromagnetic field within the
space 18 by the electrical conductivity of the support members.
[0087] If the space 18 between the inner and outer conductors is
occupied by a gas, it is desirable to avoid any gas discharge in
the space 18 in order to maximize the efficiency and uniformity of
the radiation of RF power from the RF applicator. The maximum level
of RF power that can be supplied by the RF power sources 70,74
without causing such gas discharge increases with the pressure of
the gas within the space 18. Therefore, it is desirable to maintain
the gas within the space 18 at a pressure (such as atmospheric
pressure) that is much higher than the very low pressure within the
plasma chamber.
[0088] As explained above, the first and second sealing apparatuses
52,53 abut the dielectric cover 40 such that the sealing
apparatuses, the dielectric cover and the vacuum enclosure 60 in
combination prevent fluid communication between the main portion 21
of the outer conductor and the interior 61 of the plasma chamber.
Consequently, the sealing apparatuses 52,53, the dielectric cover
40 and the vacuum enclosure 60 in combination provide a gas-tight
seal between said space and the interior of the plasma chamber so
as to enable a pressure differential between said space and the
interior of the plasma chamber. This combination 52,53,40,60
thereby enables the gas within the space 18 to be be maintained at
a pressure (such as atmospheric pressure) that is much higher than
the very low pressure within the interior of the plasma chamber.
Such higher pressure can be established, for example, by coupling
the space 18 to a gas pump or by providing an opening from the
space 18 to the ambient atmosphere, as shown in FIGS. 1 and 2, so
that the space 18 remains at ambient atmospheric pressure
regardless of the pressure within the interior of the plasma
chamber.
4. Optimizing Spatial Distribution of RF Radiation
[0089] In the following discussion, we define the "longitudinal
dimension" of the outer conductor as the dimension of the outer
conductor that extends between the first end portion 24 and the
second end portion 25, regardless of whether the outer conductor is
straight or curved, and regardless of whether the transverse cross
section of the outer conductor is rectangular, circular,
elliptical, or any other shape. We use the terms "circumferential
dimension" and "transverse dimension" to mean a dimension along the
outer surface 23 of the outer conductor that is perpendicular
(i.e., transverse) to the longitudinal dimension of the outer
conductor. The longitudinal dimension is illustrated by the axis L
in FIGS. 1,2,5 and 10-13. The circumferential dimension (or,
equivalently, the transverse dimension) is illustrated by the axis
T in FIGS. 4,6 and 10-13.
[0090] One advantage of the invention is that the spatial
uniformity of the RF power radiated from the RF applicator 10, or
the spatial uniformity of the plasma produced thereby, can be
optimized by altering the relative sizes, spacing or orientations
of apertures 30 in different portions of the main portion 21 of the
outer conductor 20.
[0091] One reason this is advantageous is that the RF
electromagnetic wave propagating through the space 18 between the
respective main portions 15, 21 of the inner and outer conductors
has a longitudinal non-uniformity in power density. Specifically,
the RF power density within the space 18 decreases progressively
with distance along the longitudinal dimension L of the RF
applicator from the one or more points on the inner and outer
conductors at which they are connected to an RF power source 70,
74.
[0092] For example, in the embodiment of FIG. 1 in which opposite
ends 12, 13 of the RF applicator 10 are connected to receive power
from two RF power sources 70, 74, the RF power density within the
space 18 is maximum near the two ends 12, 13 of the RF applicator
and progressively declines along the longitudinal dimension L to a
minimum at the center of the RF applicator. As another example, in
the embodiment of FIG. 2 in which only the first end 12 of the RF
applicator is connected to an RF power source 70 (and the second
end 13 of the RF applicator preferably is connected to a
termination impedance 79), the RF power density within the space 18
is maximum near the first end 12 of the RF applicator,
progressively declines along the longitudinal dimension toward the
center of the RF applicator, and further progressively declines
along the longitudinal dimension from the center to a minimum near
the second end 13 (i.e., the opposite end) of the RF
applicator.
[0093] To improve the spatial uniformity of RF power radiated by
the RF applicator 10, this longitudinal progressive decline in RF
power density within the space 18 between the respective main
portions 15, 21 of the inner and outer conductors can be offset by
a corresponding longitudinal progressive increase in the fraction
of RF power that is radiated through the apertures 30 in the outer
conductor. This can be accomplished if successive apertures at
progressively increasing longitudinal distance from an end of the
outer conductor that is connected to an RF power source have either
or both of: (1) a monotonically increasing fraction of the surface
area of the outer conductor that is occupied by the successive
apertures, such as by (i) monotonically increasing the area of each
successive aperture, or (ii) monotonically decreasing the spacing
between successive apertures; or (2) a monotonically increasing
angle between the long axis of the respective aperture and the
transverse or circumferential dimension T of the outer conductor
(or, equivalently, a monotonically decreasing angle between the
long axis of the respective aperture and the longitudinal dimension
L of the outer conductor).
[0094] The effect of the angle of the apertures described in the
preceding paragraph can be understood as follows. Within the main
portion 21 of the outer conductor 20, the direction of electric
current flow is essentially along the path between the first end
portion 24 (connected to the first power source 70) and the second
end portion 25 (connected either to the second power source 74, or,
if there is no second power source, preferably to a termination
impedance 79). Therefore, the electric field within each aperture
30 is oriented essentially parallel to the longitudinal dimension L
of the outer conductor.
[0095] Consequently, the RF power radiated through an individual
aperture 30 increases by a greater amount in response to increasing
the width of that aperture along the longitudinal dimension L in
comparison with increasing the width of that aperture along the
circumferential or transverse dimension T. Therefore, if one or
more apertures 30 have a non-circular cross-section, the amount of
RF power radiated through the apertures will increase as the
orientation of the apertures is changed so as to increase the angle
between the long axis of each aperture and the longitudinal
dimension L of the outer conductor, or, equivalently, so as to
decrease the angle between the long axis of each aperture and the
circumferential or transverse dimension T of the outer
conductor.
[0096] In the embodiment of FIG. 1 in which opposite ends 12, 13 of
the RF applicator 10 are connected to receive power from two RF
power sources 70, 74, the RF power density within the space 18 is
maximum near the two ends 12, 13 of the RF applicator and is
minimum at the center of the RF applicator, as stated above.
Therefore, the aforesaid monotonic change in the orientation, area
or spacing of successive apertures (i.e., increasing angle between
the long axis of successive apertures and the transverse or
circumferential dimension T of the outer conductor, increasing area
of successive apertures, decreasing spacing between successive
apertures, or otherwise increasing fraction of the surface area of
the outer conductor that is occupied by the apertures) preferably
should progress from either end of the main portion 21 of the outer
conductor toward the center of the outer conductor.
[0097] In the embodiment of FIG. 2 in which only the first end 12
of the RF applicator is connected to an RF power source 70, the RF
power density within the space 18 is maximum near the first end 12
of the RF applicator, is minimum at the second end 13 (i.e., the
opposite end) of the RF applicator, and has an intermediate value
at the center of the RF applicator. Therefore, the aforesaid
progressive change in the orientation, area or spacing of
successive apertures preferably should progress from the first end
of the main portion 21 of the outer conductor toward the center of
the outer conductor, and preferably further progress from the
center toward the second end of the main portion of the outer
conductor.
[0098] In summary, regardless of whether the RF applicator is
connected to an RF power source at both first and second ends 12,
13 as in the FIG. 1 embodiment or at only one end 12 as in the FIG.
2 embodiment, the foregoing designs for improving the spatial
uniformity of RF power radiated by the RF applicator 10 can be
characterized as follows in terms of a plurality of apertures 30 at
successive positions progressing from a first position P1 to a
second position P2 on the main portion 21 of the outer conductor.
The first and second positions are defined such that the first
position P1 is between the second position P2 and the first end
portion 24 of the outer conductor, and the second position P2 is
between the first position P1 and the center of the outer
conductor. In one embodiment, each respective aperture at said
respective positions progressing from the first position P1 to the
second position P2 has a monotonically increasing area (FIGS. 10
and 11) . Alternatively, each respective aperture at said
respective positions progressing from the first position P1 to the
second position P2 has a monotonically decreasing spacing between
adjacent apertures (FIG. 10). Alternatively, each respective
aperture at said respective positions progressing from the first
position P1 to the second position P2 has a long axis at a
monotonically decreasing angle relative to the circumferential or
transverse dimension T of the outer conductor or, equivalently, has
a long axis at a monotonically increasing angle relative to the
longitudinal dimension L of the outer conductor (FIG. 12).
[0099] A reason that the variation in area, spacing and angle of
the apertures is described above as "monotonic" rather than
progressive is to reduce the cost of manufacturing the apertures.
It is relatively expensive to manufacture a conductor in which
every aperture has a different size, spacing or orientation. The
desired longitudinal uniformity in radiated RF power can be
achieved if the variation in the apertures is step-wise rather than
continuously progressive. Specifically, a progressive change in the
area, spacing or angle of the apertures can be successfully
approximated if several consecutive apertures have the same area,
spacing and angle, and then the next several consecutive apertures
have the desired change in area, spacing or angle.
[0100] Alternatively, a spatial variation in the apertures that
improves the spatial uniformity of RF power radiated by the RF
applicator 10 can be defined in terms of differences between the
orientation, area or spacing of apertures in different portions of
the main portion 21 of the outer conductor 20.
[0101] (To avoid the awkward expression "portion of a portion", in
the following discussion we use the term "sub-portion" to refer to
a portion of the main portion 21 of the outer conductor 20.
However, the term "sub-portion" is not intended to have a different
meaning from "portion". The sub-portions need not, and typically do
not, have physical boundaries. The sub-portions are merely
different portions of the outer conductor. Furthermore, even for a
particular embodiment of an RF applicator, the boundary between the
first and second sub-portions defined below is not uniquely
determined, but can be considered to have any location for which
the relationships defined below between the first and second
plurality of apertures are fulfilled.)
[0102] FIG. 1 shows the main portion 21 of the outer conductor 20
being conceptually divided into four contiguous sub-portions
labeled 81, 82, 83, 84 extending, in that order, from the first end
portion 24 to the second end portion 25 of the outer conductor. As
stated in the preceding paragraph, the four sub-portions need not,
and typically do not, have physical boundaries. The first
sub-portion 81 extends between the second sub-portion and the first
end portion 24. The second sub-portion 82 extends between the
second sub-portion and the center of the outer conductor. The
positions of the third and fourth sub-portions 83,84 are mirror
images of the second and first sub-portions, respectively. In other
words, the fourth sub-portion 84 extends between the third
sub-portion and the second end portion 25. The third sub-portion 83
extends between the fourth sub-portion and the center of the outer
conductor.
[0103] FIG. 2 shows first, second, third and fourth sub-portions
81,82,87,88 that are defined identically to the corresponding
first, second, third and fourth sub-portions 81,82, 83,84 of FIG.
1. The third and fourth sub-portions 87,88 are numbered differently
in FIG. 2 for reasons that will be explained later.
[0104] (In FIGS. 1 and 2, the braces representing the longitudinal
length of sub-portions 81-84 and 87-88 are located in the drawings
adjacent the dielectric cover 40. This is because there is no room
in the drawings to locate the braces closer to the outer conductor
20. However, the braces are intended to point to the outer
conductor 20 that is immediately behind the dielectric cover
40.)
[0105] The apertures 30 within the first and second sub-portions
81,82 are respectively referred to as the first plurality of
apertures 31 and the second plurality of apertures 32.
[0106] FIGS. 10-12 are detail views of opposite ends of the first
and second sub-portions 81,82, in other words, the end of the first
sub-portion 81 that is closest to the first end portion 24 of the
outer conductor and the end of the second sub-portion 82 that is
closest to the center of the outer conductor. The detail views of
FIGS. 10-12 are magnified to show the differences between the area,
spacing or orientation of the first and second plurality of
apertures 31,32.
[0107] In both the FIG. 1 embodiment that is connected to RF power
at both ends 12,13 of the RF applicator and the FIG. 2 embodiment
that is connected to RF power at only one end 12, the RF power
density within the space 18 between the respective main portions
15, 21 of the inner and outer conductors declines progressively
from the first end 12 to the center of the RF applicator as stated
above. To offset this longitudinal progressive decline in RF power
density within the space 18 and thereby improve the spatial
uniformity of RF power radiated by the RF applicator 10, the
apertures 30 preferably are non-uniform in orientation, area or
spacing in accordance with either or both of the following
techniques.
[0108] In the first technique (FIGS. 10 and 11), the fraction of
the surface area of the second sub-portion 82 of the outer
conductor that is occupied by the second plurality of apertures 32
is larger than the fraction of the surface area of the first
sub-portion 81 of the outer conductor 20 that is occupied by the
first plurality of apertures 31. One possible implementation of the
first technique is that the second plurality of apertures 32 have a
larger area, either individually or on average, than the first
plurality of apertures 31 (FIGS. 10 and 11). In the embodiment of
FIG. 10, the second plurality of apertures (in the second
sub-portion 82) are larger in area than the first plurality of
apertures (in the first sub-portion 81) because they are wider in
the longitudinal dimension L of the outer conductor. In the
embodiment of FIG. 11, the second plurality of apertures are larger
in area than the apertures in first sub-portion because they are
wider in the transverse or circumferential dimension T of the outer
conductor. An alternative implementation of the first technique is
that the second plurality of apertures 32 have a smaller spacing
between adjacent apertures, either individually or on average, than
the first plurality of apertures (FIGS. 10 and 11).
[0109] In the second technique (FIG. 12), each respective aperture
30 is characterized by a respective angle at which its respective
long axis is oriented relative to the transverse or circumferential
dimension T of the second conductor, and such angles, either
individually or on average, for the second plurality of apertures
32 (in the second sub-portion 82) are smaller than such angles,
either individually or on average, for the first plurality of
apertures 31 (in the first sub-portion 81).
[0110] Equivalently, the second technique can be defined relative
to the longitudinal dimension L of the second conductor rather than
the circumferential dimension T. Considering the angle at which the
long axis of each aperture is oriented relative to such
longitudinal dimension L, such angles, either individually or on
average, for the second plurality of apertures 32 (in the second
sub-portion 82) are larger than such angles, either individually or
on average, for the first plurality of apertures 31 (in the first
sub-portion 81).
[0111] The third and fourth sub-portions of the main portion 21 of
the outer conductor 20--labeled 83,84 in FIGS. 1 and 87,88 in FIG.
2--now will be discussed.
[0112] In the embodiment of FIG. 1, each of the first and second
ends 12,13 of the RF applicator is connected to a respective RF
power source 70,74. Consequently, for purposes of our techniques
for optimizing spatial distribution of RF radiation from the RF
applicator, the second end of the RF applicator can be considered
to be a mirror image of the first end. Therefore, all preceding
statements regarding the area, spacing or angular orientation of
the apertures in the first and second sub-portions 81,82 can be
applied to the fourth and third sub-portions 84,83, respectively.
In other words, in the techniques described above for improving the
spatial uniformity of the RF power radiated by the RF applicator
10, every reference to the first sub-portion 81 can be replaced by
a reference to the fourth sub-portion 84, and every reference to
the second sub-portion 82 can be replaced by a reference to the
third sub-portion 83. In particular, the respective embodiments of
FIGS. 10-12 also apply if the first and second sub-portions 81 and
82 are replaced by the fourth and third sub-portions 84 and 83,
respectively.
[0113] In the embodiment of FIG. 2, only the first end 12 of the RF
applicator is connected to an RF power source 70. (The second end
13 of the RF applicator preferably is connected to a termination
impedance 79.) As stated above, the RF power density within the
space 18 between the respective main portions 15,21 of the inner
and outer conductors is maximum near the first end 12 of the RF
applicator, progressively declines along the longitudinal dimension
toward the center of the RF applicator, and further progressively
declines along the longitudinal dimension from the center to a
minimum near the second end 13 (i.e., the opposite end) of the RF
applicator. Consequently, for purposes of our techniques for
optimizing spatial distribution of RF radiation from the RF
applicator, the relationship between the second end and the center
is similar to the relationship between center and the first end.
Therefore, all preceding statements regarding the area, spacing or
angular orientation of the apertures in the first sub-portion 81
relative to the second sub-portion 82 can be applied to the third
sub-portion 87 relative to the fourth sub-portion 88.
[0114] In particular, in applying the first technique defined
above, the fraction of the surface area of the fourth sub-portion
88 of the outer conductor 20 that is occupied by the fourth
plurality of apertures 38 is larger than the fraction of the
surface area of the third sub-portion 87 of the outer conductor
that is occupied by the third plurality of apertures 37 (FIGS. 2
and 13). In applying the second technique, each respective aperture
is characterized by a respective angle at which its respective long
axis is oriented relative to the transverse or circumferential
dimension T of the second conductor, and such angles, either
individually or on average, for the third plurality of apertures 37
(in the third sub-portion 87) are smaller than such angles, either
individually or on average, for the second plurality of apertures
38 (in the second sub-portion 88).
[0115] It must be emphasized that the non-uniformity of the sizes,
spacings or orientations of the apertures as just described is an
optional feature of the RF applicator invention, not a requirement.
For example, the sizes, spacings and orientations of the apertures
can be uniform as shown in FIGS. 5-6 and 14-22.
[0116] Furthermore, the non-uniformity of the sizes, spacings or
orientations of the apertures as just described can be beneficial
to improve the spatial uniformity of RF power radiated by
two-conductor RF applicator designs other than the novel RF
applicator described in this patent specification. Therefore, the
techniques described in this section entitled "4. Optimizing
Spatial Distribution of RF Radiation" are a useful invention
independent of the other aspects of the RF applicator design.
5. Circumferential or Transverse Offset Between Apertures
[0117] Because each aperture 30 imposes a higher impedance to
electrical current than the conductive material surrounding the
aperture, the electrical current flowing through the outer
conductor 20 will tend to bypass the apertures if there is a
straight path for current flow along the longitudinal dimension L
of the outer conductor that is not interrupted by any apertures, as
in the embodiment of FIGS. 5 and 6. This would undesirably reduce
the electric field in the apertures and thereby reduce the amount
of RF power radiated from the apertures.
[0118] (This problem would not be significant in the limited
situation in which all the apertures are very narrow and are
oriented parallel to the longitudinal dimension L of the outer
conductor, because such apertures would impose a relatively small
impedance to current flow along the longitudinal dimension L of the
outer conductor. However, apertures with such orientation would
radiate an undesirably low amount of RF power for the reasons
explained in the preceding section of this patent specification
entitled "4. Optimizing Spatial Distribution of RF Radiation".)
[0119] The embodiments of FIGS. 14-22 illustrate that apertures 30
at successive positions along the longitudinal dimension L of the
outer conductor 20 can be offset from each other in the transverse
or circumferential dimension T of the outer surface 23 of the outer
conductor, i.e., in the dimension along the outer surface of the
outer conductor 20 that is orthogonal to the longitudinal dimension
L. Such transverse or circumferential offset can achieve the
desired result of precluding a straight path for current flow along
the longitudinal dimension L of the outer conductor that is not
interrupted by any apertures.
[0120] FIGS. 14-17 illustrate an embodiment in which each
successive aperture along the longitudinal dimension L of the outer
conductor has a circumferential offset of 90 degrees relative to
the preceding aperture. FIGS. 16 and 17 are cross sectional views
taken through two successive apertures along the longitudinal
dimension L of the outer conductor.
[0121] FIGS. 18-22 illustrate an alternative embodiment in which
each successive aperture along the longitudinal dimension L of the
outer conductor has a circumferential offset of 60 degrees relative
to the preceding aperture. FIGS. 20-22 are cross sectional views
taken through three successive apertures along the longitudinal
dimension L of the outer conductor.
[0122] The transverse or circumferential offset of the apertures as
just described can be beneficial to improve the efficiency of
two-conductor RF applicator designs other than the novel RF
applicator described in this patent specification. Therefore, the
techniques described in this section entitled "5. Circumferential
or Transverse Offset between Apertures" are a useful invention
independent of the other aspects of the RF applicator design.
6. Three-Conductor RF Applicator
[0123] FIGS. 23 and 24 illustrate a transmission line RF applicator
10 according to the second aspect or second embodiment of the
invention that includes an inner conductor 14 and two outer
conductors. We refer to the two outer conductors individually as
the first outer conductor 20a and the second outer conductor 20b,
and we refer to them collectively as the two outer conductors
20.
[0124] The inner conductor 14 has a main portion 15 extending
between first and second end portions 16,17. Each respective outer
conductor 20a, 20b has a respective main portion 21a, 21b extending
between first and second end portions 24,25. (These definitions of
the respective main portions and end portions are the same as for
the first aspect or first embodiment of the invention shown in
FIGS. 1-6 and described in the preceding section of this patent
specification entitled "1. Two-Conductor RF applicator", so they
are not labeled in FIG. 23.)
[0125] We refer to the RF applicator 10 as having opposite first
and second ends 12,13, such that the first end 12 of the RF
applicator is adjacent the respective first end portions 16, 24 of
the inner and outer conductors, and the second end 13 of the RF
applicator is adjacent the respective second end portions 17,25 of
the inner and outer conductors.
[0126] The main portion 15 of the inner conductor is positioned
between, and spaced away from, the respective main portions 21a,
21b of the first and second outer conductors 20a, 20b. The
respective first end portions 24 of each of the two outer
conductors 20 are electrically connected together (shown
schematically in FIG. 23 by first electrical connection 26).
Likewise, the respective second end portions 25 of each of the two
outer conductors are electrically connected together (shown
schematically in FIG. 23 by second electrical connection 27).
[0127] Optionally but preferably, the main portions of the inner
and outer conductors are arranged symmetrically such that the main
portion 15 of the inner conductor 14 is midway between the
respective main portions 21 of the two outer conductors 20, and the
respective main portions of the two outer conductors are either
identical or are mirror images of each other, by which we mean they
are symmetrical relative to the main portion of the inner
conductor.
[0128] The main portion 21a, 21b of each respective outer conductor
20a, 20b includes a plurality of apertures 30 that extend between
the respective inner and outer surfaces 22,23 of the respective
main portion of the respective outer conductor. The inner surface
22 faces the main portion 15 of the inner conductor. In embodiments
that include a dielectric cover 40 as described above under the
heading "3. Dielectric Cover and Dielectric Between Conductors",
the outer surface 23 of the main portion of each respective outer
conductor 21a, 21b faces the inner surface 44 of the main portion
41 of the dielectric cover.
[0129] In operation, when the output of an RF power source 70,74 is
connected between the inner conductor 14 and the two outer
conductors 20, an RF electromagnetic wave propagates through the
space 18 between the main portions 15,21 of the inner and outer
conductors. A portion of the RF power in this electromagnetic wave
radiates from the apertures 30, thereby radiating RF power outside
the RF applicator.
[0130] If the RF applicator 10 is within the vacuum enclosure 60 of
a plasma chamber as shown in FIG. 23, the RF power radiated by the
RF applicator will be absorbed by the gases and plasma within the
plasma chamber and thereby excite the gases to a plasma state or
sustain an existing plasma.
[0131] The invention is especially advantageous for use in a plasma
chamber 60 that processes two workpieces simultaneously. Because
the respective main portions 21 of the two outer conductors 20 face
opposite directions, the RF applicator 10 radiates RF power with a
bidirectional radiation pattern. Therefore, an RF applicator 10
according to the invention can be positioned between two workpieces
62 within a plasma chamber 60 as shown in FIG. 23 so as to provide
equal plasma densities adjacent the two workpieces.
[0132] As in the previously discussed embodiments of FIGS. 1-22,
multiple RF applicators 10 according to the present embodiment
having two outer conductors 20a, 20b can be positioned within the
vacuum enclosure of the plasma chamber so as to distribute the RF
power over a wider area than a single RF applicator. For example,
the multiple RF applicators 10 can be spaced apart within a
geometric plane that is equidistant between the two workpieces.
[0133] In addition to radiating RF power through the apertures 30
as described above, the RF applicator 10 will radiate RF power
through the open sides between the two outer conductors if the
transverse width of the main portion of each outer conductor is
comparable to or less than the spacing between the respective main
portions of the two outer conductors. Conversely, RF radiation in
this direction will be minimal if the transverse width of the main
portion of each outer conductor is at least two times the spacing
between the respective main portions of the two outer conductors.
This is preferred to facilitate control of the spatial distribution
of the RF radiation as described in the preceding section of this
patent specification entitled "4. Optimizing Spatial Distribution
of RF Radiation".
[0134] The RF applicator preferably includes a dielectric cover 40
and first and second sealing apparatuses 52, 53 to prevent plasma
from entering the apertures 30. Specifically, the main portion 41
of the dielectric cover is positioned within the interior 61 of the
plasma chamber, and the respective main portions 21 of each of the
outer conductors are positioned within the main portion 41 of the
dielectric cover. The first and second sealing apparatuses 52, 53
respectively abut the first and second end portions 42, 43 of the
dielectric cover. The first and second sealing apparatuses, the
dielectric cover and the vacuum enclosure 60 in combination prevent
fluid communication between the interior of the plasma chamber and
the respective main portions of the first and second outer
conductors. Further details regarding the dielectric cover and
sealing member are the same as explained in the preceding section
of this patent specification entitled "3. Dielectric Cover and
Dielectric Between Conductors".
[0135] The invention does not require the inner and outer
conductors 14, 20 to have any specific shapes. In FIGS. 23 and 24,
the main portion 15 of the inner conductor is illustrated as having
a rectangular cross section, but it alternatively can have a
circular cross section as shown in FIG. 25. In FIGS. 23 and 24, the
main portion 21a, 21b of each of the two outer conductors is
illustrated as having a rectangular cross section. FIG. 25
illustrates an alternative design in which the main portion 21a,
21b of each outer conductor has an arcuate cross section, and the
main portion 41 of the dielectric cover 40 has an elliptical cross
section.
[0136] The features, design considerations, and advantages of the
invention described above under the headings "2. Connections to RF
Power Source", "3. Dielectric Cover and Dielectric Between
Conductors" and "4. Optimizing Spatial Distribution of RF
Radiation" remain applicable to this second aspect or embodiment of
the invention having two outer conductors.
* * * * *